# std::ranges::find, std::ranges::find_if, std::ranges::find_if_not

From cppreference.com
< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
| (1) |
template< std::input_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T*> constexpr I find( I first, S last, const T& value, Proj proj = {} ); |  | (since C++20) (until C++26)
template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj> > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T*> constexpr I find( I first, S last, const T& value, Proj proj = {} ); |  | (since C++26)
| (2) |
template< ranges::input_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr ranges::borrowed_iterator_t<R> find( R&& r, const T& value, Proj proj = {} ); |  | (since C++20) (until C++26)
template< ranges::input_range R, class Proj = std::identity, class T = std::projected_value_t<ranges::iterator_t<R>, Proj> > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr ranges::borrowed_iterator_t<R> find( R&& r, const T& value, Proj proj = {} ); |  | (since C++26)
template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr I find_if( I first, S last, Pred pred, Proj proj = {} ); | (3) | (since C++20)
template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred > constexpr ranges::borrowed_iterator_t<R> find_if( R&& r, Pred pred, Proj proj = {} ); | (4) | (since C++20)
template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr I find_if_not( I first, S last, Pred pred, Proj proj = {} ); | (5) | (since C++20)
template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred > constexpr ranges::borrowed_iterator_t<R> find_if_not( R&& r, Pred pred, Proj proj = {} ); | (6) | (since C++20)

Returns the first element in the range `[`first`,` last`)` that satisfies specific criteria:

1) `find` searches for an element equal to value.
3) `find_if` searches for an element for which predicate pred returns true.
5) `find_if_not` searches for an element for which predicate pred returns false.
2,4,6) Same as (1,3,5), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | the range of elements to examine
r | - | the range of the elements to examine
value | - | value to compare the elements to
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

Iterator to the first element satisfying the condition or iterator equal to last if no such element is found.

### Complexity

At most last - first applications of the predicate and projection.

### Possible implementation

find
    struct find_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S,
                 class Proj = std::identity,
                 class T = std::projected_value_t<I, Proj>>
        requires std::indirect_binary_predicate
                     <ranges::equal_to, std::projected<I, Proj>, const T*>
        constexpr I operator()(I first, S last, const T& value, Proj proj = {}) const
        {
            for (; first != last; ++first)
                if (std::invoke(proj, *first) == value)
                    return first;
            return first;
        }

        template<ranges::input_range R, class T, class Proj = std::identity>
        requires std::indirect_binary_predicate<ranges::equal_to,
                     std::projected<ranges::iterator_t<R>, Proj>, const T*>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, const T& value, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), value, std::ref(proj));
        }
    };

    inline constexpr find_fn find;
find_if
    struct find_if_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
        constexpr I operator()(I first, S last, Pred pred, Proj proj = {}) const
        {
            for (; first != last; ++first)
                if (std::invoke(pred, std::invoke(proj, *first)))
                    return first;
            return first;
        }

        template<ranges::input_range R, class Proj = std::identity,
                 std::indirect_unary_predicate
                     <std::projected<ranges::iterator_t<R>, Proj>> Pred>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, Pred pred, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
        }
    };

    inline constexpr find_if_fn find_if;
find_if_not
    struct find_if_not_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
        constexpr I operator()(I first, S last, Pred pred, Proj proj = {}) const
        {
            for (; first != last; ++first)
                if (!std::invoke(pred, std::invoke(proj, *first)))
                    return first;
            return first;
        }

        template<ranges::input_range R, class Proj = std::identity,
                 std::indirect_unary_predicate
                     <std::projected<ranges::iterator_t<R>, Proj>> Pred>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, Pred pred, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
        }
    };

    inline constexpr find_if_not_fn find_if_not;

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (1,2)

### Example

Run this code

    #include <algorithm>
    #include <cassert>
    #include <complex>
    #include <format>
    #include <iostream>
    #include <iterator>
    #include <string>
    #include <vector>

    void projector_example()
    {
        struct folk_info
        {
            unsigned uid;
            std::string name, position;
        };

        std::vector<folk_info> folks
        {
            {0, "Ana", "dev"},
            {1, "Bob", "devops"},
            {2, "Eve", "ops"}
        };

        const auto who{"Eve"};
        if (auto it = std::ranges::find(folks, who, &folk_info::name); it != folks.end())
            std::cout << std::format("Profile:\n"
                                     "    UID: {}\n"
                                     "    Name: {}\n"
                                     "    Position: {}\n\n",
                                     it->uid, it->name, it->position);
    }

    int main()
    {
        namespace ranges = std::ranges;

        projector_example();

        const int n1 = 3;
        const int n2 = 5;
        const auto v = {4, 1, 3, 2};

        if (ranges::find(v, n1) != v.end())
            std::cout << "v contains: " << n1 << '\n';
        else
            std::cout << "v does not contain: " << n1 << '\n';

        if (ranges::find(v.begin(), v.end(), n2) != v.end())
            std::cout << "v contains: " << n2 << '\n';
        else
            std::cout << "v does not contain: " << n2 << '\n';

        auto is_even = [](int x) { return x % 2 == 0; };

        if (auto result = ranges::find_if(v.begin(), v.end(), is_even); result != v.end())
            std::cout << "First even element in v: " << *result << '\n';
        else
            std::cout << "No even elements in v\n";

        if (auto result = ranges::find_if_not(v, is_even); result != v.end())
            std::cout << "First odd element in v: " << *result << '\n';
        else
            std::cout << "No odd elements in v\n";

        auto divides_13 = [](int x) { return x % 13 == 0; };

        if (auto result = ranges::find_if(v, divides_13); result != v.end())
            std::cout << "First element divisible by 13 in v: " << *result << '\n';
        else
            std::cout << "No elements in v are divisible by 13\n";

        if (auto result = ranges::find_if_not(v.begin(), v.end(), divides_13);
            result != v.end())
            std::cout << "First element indivisible by 13 in v: " << *result << '\n';
        else
            std::cout << "All elements in v are divisible by 13\n";

        std::vector<std::complex<double>> nums{{4, 2}};
        #ifdef __cpp_lib_algorithm_default_value_type
            // T gets deduced in (2) making list-initialization possible
            const auto it = ranges::find(nums, {4, 2});
        #else
            const auto it = ranges::find(nums, std::complex<double>{4, 2});
        #endif
        assert(it == nums.begin());
    }

Output:

    Profile:
        UID: 2
        Name: Eve
        Position: ops

    v contains: 3
    v does not contain: 5
    First even element in v: 4
    First odd element in v: 1
    No elements in v are divisible by 13
    First element indivisible by 13 in v: 4

### See also

ranges::adjacent_find (C++20) | finds the first two adjacent items that are equal (or satisfy a given predicate) (algorithm function object)
ranges::find_end (C++20) | finds the last sequence of elements in a certain range (algorithm function object)
ranges::find_first_of (C++20) | searches for any one of a set of elements (algorithm function object)
ranges::mismatch (C++20) | finds the first position where two ranges differ (algorithm function object)
ranges::search (C++20) | searches for the first occurrence of a range of elements (algorithm function object)
findfind_iffind_if_not (C++11) | finds the first element satisfying specific criteria (function template)

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